1. Field of the Invention
The present invention relates to a magnetic toner to be used in a method such as electrophotography, an electrostatic recording method, or a magnetic toner jet recording method.
2. Description of the Related Art
In recent years, toner has been required to be able to correspond to increases in speed and image quality of an image forming apparatus of an electrophotographic type, such as a copying machine or a printer. In addition, an environment in which the toner is used has been diversified, and the toner has been required to be able to provide a stable image even when used in various environments.
As a developing method to be employed in the image forming apparatus, a one-component developing method using a developing device having a simple structure is preferably used from the viewpoints of less trouble, a longer lifetime, and easier maintenance.
As the one-component developing method, some methods are known. One of those methods is a jumping developing method using a magnetic toner (hereinafter also referred to simply as “toner”) including magnetic toner particles (hereinafter also referred to simply as “toner particles”) containing magnetic iron oxide particles. The jumping developing method is a method involving allowing the magnetic toner charged by triboelectric charging with a toner carrying member to fly and adhere onto a surface of an electrostatic latent image bearing member (electrophotographic photosensitive member or the like) by using a developing bias, to develop (visualize) an electrostatic latent image (electrostatic charge image) on the electrostatic latent image bearing member. The jumping developing method is widely put into practical use from the viewpoints of easy conveyance control of the magnetic toner and less contamination in the image forming apparatus.
When a content of the magnetic iron oxide particles in the toner particles is reduced, a magnetic brush on the toner carrying member can be reduced in height and uniform magnetic brush formation can be achieved, and thus the magnetic toner tends to cause less tailing and scattering, and provide satisfactory image quality. In addition, the reduction in content of the magnetic iron oxide particles is also advantageous from the viewpoint of reducing a toner consumption amount, because an image can be formed without using unnecessary toner.
From such viewpoints, the magnetic toner has been required to achieve a reduction in content of the magnetic iron oxide particles in the toner particles.
In addition, a binder resin in the toner particles has great influences on the above-mentioned characteristics of the toner. Examples of the binder resin in the toner particles include a polystyrene resin, a styrene-acrylic resin, a polyester resin, an epoxy resin, and a polyamide resin. Of those resins, a polyester resin, which exhibits excellent low-temperature fixability and the like, has recently attracted attention.
As described above, the environment in which the toner is used has been diversified in recent years. Now, adaptability of the toner to various environments is focused, and one factor having particularly great influences among environmental factors is humidity. The humidity has influences on a charge amount and charge amount distribution of the toner, causes variations in developability, and in addition, has a great influence on transferability.
In a transfer step of transferring the toner from the surface of the electrostatic latent image bearing member onto paper, a charge having a polarity opposite to that of the toner is imparted to the paper from its back surface, to charge the surface of the paper with a polarity opposite to that of the toner, to thereby transfer the toner. At this time, while only the surface of the paper is intended to be charged, the charge passes the paper from its back surface to its front surface depending on a kind of the paper or humidity and charges also the toner on the surface of the electrostatic latent image bearing member in some cases. In those cases, the toner is charged with a polarity opposite to its original polarity. Such phenomenon is called “transfer penetration.” When the transfer penetration occurs, the toner may be prevented from being transferred onto the paper and remain on the surface of the electrostatic latent image bearing member, and a toner image may be disturbed at the time of transferring, resulting in blank areas or unevenness in the toner image transferred onto the paper. In addition, a half-tone image or the like may be coarse. Such phenomenon is particularly remarkable when the image is output in a high-temperature and high-humidity environment.
Japanese Patent Application Laid-Open Nos. 2000-214625 and 2005-37744 each disclose a technology for solving the problem by externally adding the magnetic iron oxide particles to the toner particles.
In addition, Japanese Patent Application Laid-Open No. 2005-157318 discloses a technology involving reducing the content of the magnetic iron oxide particles in the toner particles as compared to that in the related art and controlling a saturated magnetization amount and dielectric loss tangent of the magnetic iron oxide particles.
In addition, Japanese Patent Application Laid-Open Nos. 2005-181759 and 2007-133391 each disclose that a resin obtained by introducing a long-chain alkyl group in a polyester resin is used in the toner particles in order to improve dispersibility of wax in the toner particles.
However, the technologies disclosed in Japanese Patent Application Laid-Open Nos. 2000-214625 and 2005-37744 each have an insufficient effect of suppressing the transfer penetration in a high-humidity environment, in which the transfer penetration is liable to occur.
In addition, in the technology disclosed in Japanese Patent Application Laid-Open No. 2005-157318, the magnetic iron oxide particles are liable to be unevenly distributed in the toner particles. In addition, even if the magnetic iron oxide particles are uniformly dispersed in the toner particles, electrical resistance is liable to vary in the toner particles between a portion in which larger magnetic iron oxide particles are present and a portion in which smaller magnetic iron oxide particles are present, unless the magnetic iron oxide particles have a sharp particle size distribution. As a result, the technology has an insufficient effect of suppressing the transfer penetration in use in an environment in which the transfer penetration is liable to occur.
In addition, Japanese Patent Application Laid-Open Nos. 2005-181759 and 2007-133391 do not make detailed investigations on the magnetic iron oxide particles.
As described above, there has been a demand for a magnetic toner using a polyester resin exhibiting excellent low-temperature fixability and the like, and concurrently achieving a reduction in content of the magnetic iron oxide particles in the toner particles from the viewpoints of magnetic brush height reduction and uniform magnetic brush formation.
However, when the content of the magnetic iron oxide particles is reduced, a problem of poor dispersibility of the magnetic iron oxide particles in the toner particles is liable to occur. As a result, electrical resistance is liable to vary in the toner particles between a portion in which the magnetic iron oxide particles are present and a portion in which the magnetic iron oxide particles are absent, and thus the transfer penetration is liable to occur.
In addition, as a result of investigations made by the inventors of the present invention, the polyester resin has been found to be more liable to cause the transfer penetration than other resins.
Japanese Patent Application Laid-Open Nos. 2000-214625, 2005-37744, 2005-157318, 2005-181759, and 2007-133391 do not make investigations on the problem of the transfer penetration in the magnetic toner using as the binder resin in the toner particles a polyester resin and achieving a reduction in content of the magnetic iron oxide particles in the toner particles.